Marie Antoine de Saint-Cyr was born in 1753, to the impoverished Marquis de Saint-Cyr, and his wife Marie Josephine de Clemenceau, daughter of the Vicomte de Brouille. She is the youngest daughter, with an older sister Élisabeth-Athénaïs , and a younger brother Charles-Sébastien. After the death of her maternal grand-father, the Vicomte de Brouille, the family inherited much of his wealth. They relocated to Paris, leaving the family Chateau de Saint-Cyr, and spent a great fortune renovating the petit Palais de Saint-Cyr, located in the heart of Paris, on the Ile Saint Louis. With their remaining wealth, they launched themselves into court life at the glittering Court of Versailles, and the young Mademoiselle de Saint-Cyr was presented to the royal family soon after.

She was admired by many, but she had eyes only for the young Prince de Carpègne, only a few years her senior, and heir to the Duchy of Dampierre. But her father had other plans. Recently elevated to a Duchy, the new Duc de Montausier and his family were received by almost every respectable family at Versailles. He heard rumours of the Prince de Fenetrange attempting to marry off his youngest daughter to the aging Duc de Rochefort, heir to the largest fortune outside of the royal family. The Duc quickly introduced his charming daughter, and the Duc de Rochefort was enraptured by her. They were soon married, but the new Duchesse de Rochefort wasn't content in this marriage of convenience. She spent most of her time receiving, and being recevied by the King's new mistress, the Comtesse du Barry, who had arrived at court soon after their marriage.

The Duchesse de Rochefort, now enjoying her newly acquired wealth, spent frivolously. She had the family Chateau de Saint-Cyr returned to it's former splendour, and spent a fortune renovating the grand Chateau de Rochefort on the west coast. She was rumoured to have so many gowns that she could wear 3 everyday for a year and never wear the same gown twice. She enjoyed the new fashion trend for wide panniers and towering pouffes, and was the leader in this new revolution of Haute Couture. She spent thousands redecorating her luxurious apartment at Versailles, acquired with the help of Du Barry. It contained 5 spacious rooms, and overlooked the sunny South Parterre and Orangerie.

But tragedy struck when the Duc de Montausier died in 1770 from smallpox. The Dowager Duchesse then moved away from Versailles to Saint-Cyr, along with her eldest daughter, leaving the new Duc de Montausier in the care of his older sister, the Duchesse. He married in 1771, to Mademoiselle Elisabeth de Rougé, eldest daughter of the Marquis du Plessis. Marie was her Maid of Honour, and welcomed her sister-in-law into the family. Soon after the wedding, the new Duc and Duchesse left Versailles for their new home, Chateau de Saint-Cyr, which now belonged to Charles.

She was heartbroken when she discovered the Prince de Carpègne was soon to be married aswell. She embarked on a scandalous affair, and relied on the power of Du Barry to allow her to do so. Soon after, the Princesse de Carpegne was discovered dead in her apartment, chopped up into seven-ish pieces, with a random man on top of her, also dead. The grieving Prince was most distraught, and was heavily comforted by the Duchesse. Meanwhile, the Dowager Duchesse de Rochefort had plans to annul the marriage of her son as she was afraid her son's reputation would take damage